1. Field of the Invention
The present invention relates to a controlled-output amplifier whose output power may be set to any of a number of predetermined levels. More specifically, it relates to a controllable-gain amplifier whose output is controlled by a servo loop that includes a half-wave rectifier for sensing the output power and whose gain is controllable to maintain a desired power level over a wide range of ambient temperature.
2. Discussion of the Prior Art
Controlled-output amplifiers known in the prior art include a controllable-gain amplifier stage connected in a servo or feedback loop in which the output power is sensed and compared with a reference. The resulting difference or error signal is used to control the gain of the amplifier stage, thereby maintaining the amplifier output at the desired power level. Power sensing is usually accomplished by a diode, frequently referred to as a "detector diode," which is connected to a capacitor to form a half-wave rectifier. However, diodes possess certain characteristics which may produce adverse effects in some applications.
First, if the monitored power levels are relatively low (e.g. on the order of tens of milliwatts or less), the forward voltage drop of the detector diode is large in comparison to the rectified signal and variations in the forward drop may therefore introduce significant errors into the rectified signal.
Unfortunately, the forward voltage drop of a diode is highly temperature dependent. For silicon diodes the forward drop typically varies on the order of -2 mV/.degree.C. In addition, the rate of change of the forward drop with respect to temperature is dependent on the bias current flowing through the diode. Thus, over a wide ambient temperature range, the forward voltage drop will vary significantly.
Third, diodes typically provide very limited dynamic ranges. In other words, a diode can sense signals only within a narrow amplitude range without being driven into saturation. In applications where the sensed signal may vary by an order of magnitude or more, the diode may saturate, thereby introducing errors into the rectified signal.
A conventional solution for minimizing the problems with forward voltage drop and drift described above is the application of temperature compensation to the detector diode. In one form of compensation, a "compensation diode" is connected so that its forward drop biases the detector diode at the threshhold of conduction and thus effectively cancels the forward voltage drop of the detector diode. Assuming that the two diodes have "matched" temperature characteristics, their forward voltage drops will substantially track each other as the temperature varies and the detector diode will thereby remained biased at the threshhold of conduction.
There are several disadvantages associated with this type of temperature compensation. First, the detector and compensation diodes must have nearly identical temperature characteristics to ensure close tracking between their forward voltage drops. Such matched diodes are relatively expensive components; typically they must be tested for close matching before they can be assembled in a desired circuit. Second, the two diodes must be located in close thermal proximity to each other in order to minimize any difference in ambient temperature. If a significant temperature difference exists, the forward voltage drops of the two diodes will not be the same and the forward voltage drop of the detector diode will not be effectively cancelled as the temperature varies, resulting in temperature-related variations in the output of the detector diode.